A lightweight polyfill library for Promise-based WebExtension APIs in Chrome

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README.md

WebExtension `browser` API Polyfill

This library allows extensions written for the Promise-based WebExtension/BrowserExt API being standardized by the W3 Browser Extensions group to be used without modification in Google Chrome.

Building

To build, assuming you're already installed node >= 6 and npm, simply run:

npm install
npm run build
npm run test

This will install all the npm dependencies and build both non-minified and minified versions of the final library, and output them to dist/browser-polyfill.js and dist/browser-polyfill.min.js, respectively, and finally executes the unit tests on the generated dist files.

Basic Setup

In order to use the polyfill, it must be loaded into any context where browser APIs are accessed. The most common cases are background and content scripts, which can be specified in manifest.json:

{
  // ...

  "background": {
    "scripts": [
      "browser-polyfill.js",
      "background.js"
    ]
  },

  "content_scripts": [{
    // ...
    "js": [
      "browser-polyfill.js",
      "content.js"
    ]
  }]
}

For HTML documents, such as browserAction popups, or tab pages, it must be included more explicitly:

<!DOCTYPE html>
<html>
  <head>
    <script type="application/javascript" src="browser-polyfill.js"></script>
    <script type="application/javascript" src="popup.js"></script>
  </head>
  <!-- ... -->
</html>

And for dynamically-injected content scripts loaded by tabs.executeScript, it must be injected by a separate executeScript call, unless it has already been loaded via a content_scripts declaration in manifest.json:

browser.tabs.executeScript({file: "browser-polyfill.js"});
browser.tabs.executeScript({file: "content.js"}).then(result => {
  // ...
});

Using the Promise-based APIs

The Promise-based APIs in the browser namespace work, for the most part, very similarly to the callback-based APIs in Chrome's chrome namespace. The major differences are:

Rather than receiving a callback argument, every async function returns a Promise object, which resolves or rejects when the operation completes.
Rather than checking the chrome.runtime.lastError property from every callback, code which needs to explicitly deal with errors registers a separate Promise rejection handler.
Rather than receiving a sendResponse callback to send a response, onMessage listeners simply return a Promise whose resolution value is used as a reply.
Rather than nesting callbacks when a sequence of operations depend on each other, Promise chaining is generally used instead.
For users of an ES7 transpiler, such as Babel, the resulting Promises are generally used with async and await, rather than dealt with directly.

Examples

The following code will retrieve a list of URLs patterns from the storage API, retrieve a list of tabs which match any of them, reload each of those tabs, and notify the user that is has been done:

browser.storage.get("urls").then(({urls}) => {
  return browser.tabs.query({url: urls});
}).then(tabs => {
  return Promise.all(
    Array.from(tabs, tab => browser.tabs.reload(tab.id)));
  );
}).then(() => {
  return browser.notifications.create({
    type: "basic",
    iconUrl: "icon.png",
    title: "Tabs reloaded",
    message: "Your tabs have been reloaded",
  });
}).catch(error => {
  console.error(`An error occurred while reloading tabs: ${error.message}`);
});

Or, using an async function:

async function reloadTabs() {
  try {
    let {urls} = await browser.storage.get("urls");

    let tabs = await browser.tabs.query({url: urls});

    await Promise.all(
      Array.from(tabs, tab => browser.tabs.reload(tab.id)));
    );

    await browser.notifications.create({
      type: "basic",
      iconUrl: "icon.png",
      title: "Tabs reloaded",
      message: "Your tabs have been reloaded",
    });
  } catch (error) {
    console.error(`An error occurred while reloading tabs: ${error.message}`);
  }
}

It's also possible to use Promises effectively using two-way messaging. Communication between a background page and a tab content script, for example, looks something like this from the background page side:

browser.tabs.sendMessage("get-ids").then(results => {
  processResults(results);
});

And like this from the content script:

browser.runtime.onMessage.addListener(msg => {
  if (msg == "get-ids") {
    return browser.storage.get("idPattern").then(({idPattern}) => {
      return Array.from(document.querySelectorAll(idPattern),
                        elem => elem.textContent);
    });
  }
});

or:

browser.runtime.onMessage.addListener(async function(msg) {
  if (msg == "get-ids") {
    let {idPattern} = await browser.storage.get("idPattern");

    return Array.from(document.querySelectorAll(idPattern),
                      elem => elem.textContent);
  }
});

Or vice versa.